You know that nitrate can have many many reasons.
First important is your waterchange every week,up to 50%.
Then the feeding and overcrowding(take out some fish i would say for a start).
In your country you use a lot of technichal stuff,and that is not always perfect.
Try to make a biological filter.
We,in my club in Holland make everything by ourselfs,including the fishtanks in any size you want.
but we are well known for our technical systemes,filtering,light and so on.
Lets say you have a tank with a front size of 1.50m and something like 90 gallons,then we make an almost same aquarium(a little bit smaller)under the tank with several department(including CO2 if needed).
The water runs out of the main-tank into this filter with help of a radiatorpump(stong enough for this tank of course).
On the other side it will go up thrue a compartment with lavastone,and then back into the maintank.
In this filtertank with a lot of different filterstuff(you can hide your heater,thermometer and other things there to)will bacteria breed there and brake off the nitrates.
Of course this need some time to develop.
give a new tank chance to develop,first plants,after two weeks maybe some fish,but never to many,this is always bad.
Don't forget that even the biggest tank is nothing in compare with nature.
Esspecially the rock-tanks without plants can have nitrate problems.
Maybe you should first change water every 2 days,and then see what happens,but i am sure there is some other problem in your tank that makes the nitrate go up so much.
Find it,maybe wood that is rotting or something else,i cannot see that from here.
I can only recommend a biologig filtersystem,sure for Lake Tanganjika and Malawi.
Good luck! Willy Bijker,Holland
Willy,Male,Holland,Dwarfs,West-Africa,Central-Amerika,South-America,Tanganjika,Author of articles,Give lectures to.